In tractor trailers, the control valves serve the purpose of controlling the pressure of a trailer brake system in accordance with the pressure in the brake circuit of the tractor or motor vehicle.
The pressure control takes place in such a way that the compressed air flows into the control chamber of the trailer control valve when operating the motor vehicle brake valves. This causes the relay piston, which is functionally interconnected to a combination inlet and outlet valve, to move downwardly for initial closing of the outlet valve which is directly connected through a vent valve to atmosphere. As the relay piston moves further downwardly, the inlet valve which is interposed between a pressure supply chamber and a main working chamber is opened.
The compressed air in the inlet pressure supply chamber which is received from a suitable pressure air supply tank, is conveyed to the control chamber of the trailer brake valve via the main working chamber. That is, the fluid pressure line which leads to a trailer brake valve of the trailer braking system is pressurized when the inlet valve is opened. Once the modulated pressure has risen to a certain level, which is related to the motor brake control pressure and the working surfaces of the relay piston, the relay piston is moved upward by the pressure in the working chamber and the inlet valve will become closed. The trailer control valve is in a lap position. If the motor vehicle brake control pressure is released, the modulated pressure acting upon the relay piston dominates and slides the relay piston further upwardly. As the relay piston moves upwardly, the outlet valve opens and the pressure from the control chamber of the secondary trailer brake valve, as well as the fluid pressure valve leading to the trailer brake valve, is reduced by way of the control valve.
The trailer control valves should exhibit an arrangement which affects a lead or faster build-up of brake pressure in the wheel brake cylinder of the trailer as compared to the motor vehicle. After the rapid build-up of brake pressure in the wheel brake cylinder of the trailer, the motor vehicle and trailer remain straight, even in the case of a rapid deceleration.
A known trailer control valve used in system No. 973 002 601.0 of WABCO Vehicle Brakes GmbH is provided with an auxiliary control chamber to achieve the forward slip. The auxiliary control chamber is limited by a multi-stepped relay piston. A second combination inlet and outlet valve and the auxiliary control chamber are connected with the working chamber of the trailer control valve. At a given pressure load of the relay piston by control pressure, the relay piston is shifted in the direction of the working chamber. This causes the outlet valve of the first combination inlet and outlet valve which connects the working chamber with the atmosphere to be closed, and then causes the inlet valve which connects the control chamber of a secondary trailer brake valve with a pressure air supply tank to be opened. The pressure building up in the working chamber acts upon the contact working surface of the relay piston opposite to the control pressure. Simultaneously, the valve body of the second combination inlet and outlet valve is acted upon by the same pressure in the direction of the inlet valve.
If the pressure in the working chamber has risen to a point where it overcomes the force of the spring biasing the valve body of the inlet valve, the inlet valve opens and the pressure in the working chamber is conveyed to the auxiliary control chamber. The working surface of the relay piston which is in the opposite direction to the control pressure and limiting the auxiliary control chamber, is acted upon by pressure which is reduced as compared to the pressure from the working chamber. Due to the connection of the working surface, formed by the graduation of the relay piston and also acted upon by the outlet pressure, and the thus changed surface relationship, the pressure introduction into the trailer brake valve takes place according to a characteristic curve which in the beginning is very steep and becomes more level after the opening of the inlet valve of the second combination inlet and outlet valve.
If, for the purpose of reducing brake pressure in the wheel brake cylinder, the control pressure is exhausted, the relay piston is moved in the direction of the control chamber by the pressure in the working and auxiliary control chambers. The inlet valve of the first combination inlet and outlet valve closes then the outlet valve opens and the control chamber of the trailer brake valve is vented into the atmosphere via the working chamber and the vent valve of the tractor-trailer control valve. If the pressure in the working chamber of the trailer control valve has dropped so far that the pressure in the auxiliary control chamber is predominant, the outlet valve of the second combination inlet and outlet valve opens and the pressure from the auxiliary control chamber is reduced by way of the working chamber. The reduction of the pressure in the auxiliary control chamber and in the working chamber takes place in the ratio 1:1 compared to the reduction of pressure in the control chamber.
It will be appreciated that during the introduction of pressure into the auxiliary control chamber, the force of the spring which biases the inlet valve body and determines the forward lead must be overcome. However, during exhausting of the auxiliary control chamber, the spring force determining the forward lead has no influence on the brake pressure reduction in the auxiliary control chamber. Thus, the brake pressure reduction takes place according to a characteristic curve which deviates from the characteristic curve of the brake pressure build-up.
A further object of this invention is to provide a tractor-trailer brake valve for controlling the pressure in the trailer brake circuit in accordance with the pressure in the tractor brake circuit comprising, a housing having a first control inlet, a second control inlet, a fluid pressure inlet, a fluid pressure outlet and an exhaust port, a relay piston disposed within said housing and defining a main control chamber, a diaphragm piston disposed within the housing and defining first and second auxiliary working chambers, the diaphragm piston supporting a graduated tube having a first double valve body, an intermediate piston disposed with the housing and defining a main working chamber and a pressure supply chamber, the intermediate piston having a first inlet valve seat, the relay piston having an extension portion forming a first outlet valve seat, an operating piston cooperatively associated with the relay piston and defining first and second partial chambers, the operating piston is provided with a second inlet valve seat which cooperates with a second double valve body, the extension portion accommodating a fistulous insert forming a second outlet valve seat, a passageway connecting the main working chambers with the first partial chamber, a bore hole connecting the second partial chamber with an auxiliary control chamber.
The invention especially offers the advantage that the auxiliary control chamber is directly exhausted to the atmosphere so that the hysteresis is reduced substantially, and thus the responding behavior of the brake pressure control valve is improved significantly.